PURPOSE: To enhance methane fermentation capacity by attaching a spacer to an atachment and detachment flange provided to an insert port in a freely detachable manner while attaching a heating and stirring unit having a support frame, a heater and a stirrer to said spacer.

CONSTITUTION: A fermentation stock solution receives sufficient stirring and heating by a heating and stirring unit 25 to be accelerated in methane fermentation due to methane producing bacteria present in the fermentation solution 35 in a tank 20 and gradually moves from the side of a charging pipe port 42 to the side of a discharge pipe port 44 while methane gas is discharged to the space above the fermentation solution 35. The methane gas discharged to the space above the fermentation solution 35 is sent to a gas purifying and storing apparatus through a gas discharge pipe port 39. The fermentation solution 35 in the methane fermentation tank is pumped up by a pump 29 to be scattered to the inside and outside of a warm water coil 31 from an emitting pipe port 34 and heated while passes the inside or outside of the warm water coil 31 and again fallen to a suction port 33.